1. Field of the Invention
This invention relates to improved biologically pure, novel strains of quick acting general purpose bakers yeast which show good performance in sweet and regular dough systems and especially superior performance in sweet doughs, and methods of obtaining the same from existing strains of quick acting bakers yeast by hybridization via protoplast fusion of mitochondrial petite mutants of such quick acting yeast, to quick acting active dry yeast made from these strains yeast and to improved baking methods using said novel yeast strains in active dry yeast form.
2. Description of the Prior Art
The quick acting strains of the yeast Saccharomyces cerevisiae are known and are commercially available in active dry yeast form. These yeast are generally characterized by the rapid production of a relatively large amount of carbon dioxide in a given time frame when mixed with doughs which basically contain flour and water, and which usually fall into one of the general categories of sweet, lean or regular doughs. Quick acting yeast are on the average 25% faster than normal dry yeast strains. The value of these so called quick bakers yeasts are especially significant in the commercial baking field particularly when used in the active dry yeast form containing low amounts, i.e., 4 to 8 percent, of moisture. Some of the quick acting yeasts which are available are characterized by a tolerance to sweet doughs which contain substantial amounts of sugar, i.e., up to about 12 to 18% sugar. Generally speaking, these fast acting, or so called quick yeasts of commerce have only moderate activity with a regular or straight dough which contains 4 to 5% sugar and relatively poor activity in lean doughs which contain no added sugar. This recognition has led to commercial yeast producers manufacturing separate yeast products for commercial use in either sweet doughs or lean doughs.
Industrial wild type baking yeast strains, because of their lack of selectable markers cannot be differentiated on laboratory media. That is, upon hybridization of two industrial wild type baking yeast strains no selectable markers are apparent to differentiate the yeast hybrids from the parental strains. Similarly, protoplast fusion of wild type industrial yeast strains has been unsuccessful because fusion products cannot be differentiated from the parental strains. Traditionally, new industrial yeast strains were produced by hybridization. This classical genetic technique uses the yeast cell's natural sexual cycle. Under specific conditions the organism can be made to sporulate. Spore clones of opposite mating type and desired traits can be mated to form a new hybrid yeast. The use of hybridization techniques, however, has been hindered by the fact that many industrial strains have defects in their sexual cycle. Consequently, the genetic content of many strains with desired traits is difficult to recover in spore clones where it can be studied to gain insights into the strains specific characteristics.
The use of recombinant DNA techniques is another way to modify existing industrial yeast strains. Molecular biological manipulation of industrial yeast strains, however, has limited practical uses. Recombinant DNA technology is best suited for the transfer of single gene traits or traits controlled by a small number of genes.